The invention relates to a method and apparatus for the treatment of surfaces of a workpiece of carbon-containing cast iron and the resulting product.
Surface structure is of primary importance to the tribological conditions prevailing in the case of a sliding pair. Particularly in the case of gray iron cast cylinders for internal-combustion engines, the surface structure of the gray iron casting is of decisive importance with regard to low wear, reduction of friction, breaking-in behavior and gas tightness of the piston rings. For reasons of oil retention, which is an important factor in minimizing the wear of associated sliding parts moving in the area of mixed friction, so-called oil pockets must be present in the gray iron surfaces in the form of cut graphite flakes and/or machined grooves. Apart from the presence of oil, the degree of surface roughness must be low (microroughness) if friction is to be kept to a minimum. The consensus of opinion has been that optimization of the breaking-in behavior requires that the geometry of the surface be sufficiently "aggressive" to permit the parts in sliding contact to wear each other down during the breaking-in period. In the sliding pair formed by cylinder wall and piston ring, this results in good roundness coupled with good gas tightness.
Moreover, excessive surface strain hardening must be avoided to protect the piston rings in particular from undue adhesive and/or abrasive wear.
With all known finishing methods (turning, grinding, reaming) and even with the frequently practiced mechanical honing as also with electrochemical honing, the above requirements cannot be satisfactorily met in a number of respects. Due to the cutting forces in mechanical treatment, the graphite flakes become clogged and graphite may be squeezed out of the flakes to some depth. Strain hardening of the surface occurs, and this can be extensive. Because of the clogged graphite flakes, considerable surface roughness has to be generated to provide somewhat satisfactory tribological conditions. This intentional surface roughness cannot be regarded as a full substitute for open graphite flakes since it results in considerable wear as the interacting surfaces are broken in, with the oil-retaining effect then diminishing.
Crushing of the surface of the gray iron casting occurs even in electrochemical honing, in which the honing tool serves as electrode, and hence for the electrochemical removal of stock, as well as for mechanical stock removal. To maintain a given spacing between electrode and workpiece surface, the honing stones must be pressed fairly hard against the workpiece surface. Due to this contact pressure, a corresponding violent mechanical working of the surface of the workpiece takes place, and this results in a clogging of the graphite flakes and in strain hardening of the surface. Details on electrochemical honing and on the tool used with this technique may be found, for example, in the thesis "Untersuchung des Elektrochemischen Honens" by Egbert Sholz, June 22, 1968, Polytechnic Institute of Aachen.
Electrochemical honing is afflicted not only with the shortcomings described but also with additional drawbacks. For example, the tool is of complex design, and therefore expensive, since the mechanical and electrical tool components must be accommodated in the smallest possible space. A relatively small electrode surface imposes restrictions on the process parameters. High current densities and relatively high electrolyte concentrations must be employed. The choice of honing sticks is subject to severe limitations. Metallic bonds as used in diamond honing sticks cannot be employed as they would be subject to anodic metal removal. The electrolyte used in concurrent electrochemical and mechanical stock removal cannot be optimized simultaneously with respect to corrosion, wear, influencing of mechanical stock removal, tendency of the honing sticks to clog, and influencing of electrochemical stock removal. Finally, the honing sticks located in the circulating electrolyte interfere with the electrolyte flow conditions, which may result in nonuniform stock removal.
The invention described herein overcomes problems discussed above. More specifically, the invention provides a method for the treatment of surfaces of a workpiece of carbon-containing cast iron as well as an apparatus for carrying out said method whereby a surface structure of carbon-containing cast iron that is particularly well adapted to the tribological conditions prevailing in sliding pairs can be obtained economically. Also, the invention provides a cylinder for an internal-combustion engine having improved operating characteristics.
Surprisingly, it has been found that surfaces which, from the tribological point of view, are perfectly satisfactory can be produced economically by separating the electrochemical stock removal from the purely mechanical surface treatment. The above limitations to which combination tools are subject do not apply in electrochemical stock removal, which therefore permits economical electrochemical surface removal that is satisfactory from the point of view of dimensional accuracy. As a rule, this electrochemical surface removal cannot be effected in one operation down to the desired final dimensions since it is increasingly slowed down as graphite flakes become exposed and removed stock deposits on the workpiece surface. In the strictly mechanical reaming and plating of the electrolytically treated surface which follows, the protruding graphite is removed without the pockets being closed by adjacent metallic material. Moreover, what is being removed and subsequently plated on in reaming-and-plating is mainly surface constitutents of the basis metal, primarily mixed crystals. Strictly speaking, reaming-and-plating thus is not a stock-removing treatment but a reaming with simultaneous plating in which a substantial portion of the material removed is plated on to give a surface in which load-carrying level plateaus which are partly plated with graphite, mixed crystals and/or possible electrolysis residues alternate with open graphite flakes. In contrast to the conventional bright surfaces, such a surface produced in accordance with the invention has a dark, lustrous appearance.
The graphite protruding from the graphite flakes is entrained, along with the products deposited on the electrolytically treated surface, by the reaming-and-plating tool, formed preferably of planar reaming-and-plating sticks with silicon carbide in a ceramic bond and operating in an aqueous medium, and plated onto the workpiece surface.
The number of alternative steps of electrochemical stock removal and reaming-and-plating is appropriately determined by the particular requirements. An elaborate combination tool is not required in carrying out the method in accordance with the invention. Rather, tools of conventional design may be used in electrochemical stock removal and in the mechanical treatment.
Different liquids which best satisfy the individual requirements are advantageously used in electrochemical stock removal and in the mechanical treatment. In the latter, conventional liquids which result in as little strain hardening as possible may be used. On the other hand, a single liquid such as sodium nitrate, NaNO.sub.3, may be used, which will simplify the practice of the method.
The wear resistance of the workpiece surface can be enhanced. Hardening may be carried out by many different conventional methods, such as remelt hardening, induction hardening, electroerosive hardening, etc.
The apparatus in accordance with the invention may be equipped with an electrical station and a mechanical station between which the workpiece is shuttled back and forth. However, the apparatus may also incorporate a greater number of individual stations adapted to the operations which are performed there.
Other advantages include an apparatus which is particularly well suited for the finishing of blind holes. The invention is particularly advantageous for the treatment of large bores, for example, cylinder sleeves for large diesel engines.
A cylinder with a bore produced by the method in accordance with the invention has a number of properties which distinguish it over cylinders used heretofore. Surprisingly, that bore has excellent breaking-in properties and a long service life even though the geometry of its surface is by no means "aggressive". The good properties of the bore, which might also be formed by a rolling treatment, are presumably due to the graphite laminated onto it and/or to the M.sub.23 C.sub.6 mixed crystals and any further plated-on residues, which during the breaking-in period act as laps for the associated sliding part, in this case, the piston ring or rings. When broken in, the interacting surfaces will be supplied with lubricant from the finely disseminated open graphite flakes and the likewise finely disseminated oil-retaining minute pits present between the plateaus. In the case of conventional surfaces from which stock has been mechanically removed or which have been honed, only a small portion of the surface initially bears the load, and that portion increases in a nonpredictable manner during the breaking-in period. In the broken-in condition, many graphite flakes are plugged up, and only tiny pits of random distribution and size are available as oil reservoirs.
The invention is suited for the mechanical treatment of practically all surfaces of carbon-containing cast-iron workpieces with which another part is in sliding contact. It is particularly well-suited for the mechanical treatment of gray iron cast cylinder bores in internal-combustion engines. In that application, it offers substantial advantages such as: longer service life of the internal-combustion engine; better gas tightness between piston ring and cylinder; reduced oil consumption; and fewer piston rings, and hence reduced friction and reduced fuel consumption. Notwithstanding the prevailing view, these advantages are obtained even though no surface of a particularly "aggressive" geometry is available for breaking in.
The above discussion of the invention relates to some general advantages of the invention over what has existed heretofore. Other advantages will be appreciated from the detailed discussion of the preferred embodiment which follows.